Measles is a constant threat not only in developing but also in developed countries. In the USA outbreaks in inner-cities of large metropolitan areas are perennial. Part of the problem is low immunization coverage as well as importation of measles from nearby developing countries in which immunization represents a logistical as well as scientific problem. For several years attention was directed to a novel kind of measles vaccines produced in a continuous line of human diploid lung cells as opposed to conventional measles vaccines which are produced in chick embryo fibroblasts. A study was performed in 2,097 Haitian children comparing the human diploid cell produced "Edmonston-Zagreb" vaccine with the conventional chick embryo derived 'Schwarz' measles vaccine. Two strengths of the vaccines were used, 10 or 100 times higher than that of conventional vaccines. It was found that children who received the Edmonston-Zagreb vaccine at 6 or 8 months of life had significantly higher seroprevalence rates at 1 and 2 years of age than children immunized with the Schwarz vaccine. Also, recipients of the Edmonston-Zegreb vaccine were more likely to have post-vaccination antibody titers equal or more than 200 mill-International Units considered the minimum antibody level to afford protection upon exposure of the child to wild measles virus. Such findings are essential to provide guidance for optimal immunization regimens in countries where measles is endemic, causes severe disease in infants below 9 months of age, and serves as focus for spread of the infection throughout the hemisphere.